In an open angle glaucoma, the ocular hypertension is due to an increase of the resistance to the aqueous humor outflow at the trabeculum level. Pigmentary and pseudoexfoliative types glaucoma are also subject to this effect. The Laser Trabecular Dissection (LTD) has had excellent results in treating the above-mentioned types of glaucoma. The Laser Trabecular Dissection (LTD) has not yet been totally proven in congenital, neovascular or any other type of glaucoma in which a fibrovascular or mesoderm tissue is placed between the trabeculum and the anterior chamber.
In U.S. Pat. No. 5,549,598 to O'Donnell a method is disclosed for controlling open angle glaucoma by surgery. A laser is used in the surgery to reduce the thickness of the trabecular meshwork and tissue around Schleman's Canal to increase filtration of the aqueous human and thereby control the open angle glaucoma. The surgical treatment is used as an alternative to treatment with different drugs, which potentially have adverse side affects. In the patentee's description, reference is made to surgical treatment, described as the laser trabeculoplasty technique with an argon laser, and there is an analysis of its action and clinical results.
The O'Donnell patent discusses that the pressure decrease may be transient and, in such cases, he goes on to filtrating techniques within which the trabeculectomy is the selected one, and then he goes on with the description of this technique.
In U.S. Pat. No. 5,370,641 to O'Donnell, energy in the ultraviolet wavelenght under 230 nonometers is used to perform the trabeculodissection. The preferred embodiment of this patent utilizes a galvanometric scanning system (GSS) rather than a variable opening (iris diaphragm) delivery system. Examples given of the GSS are the Compak 200 Mini excimer and the Laserharmonic, both manufactured by Lasersight, Inc. (Orlando, Fla.). As described, the advantages of a galvonometric scanning delivery system are rooted in the anatomy of the portion of the eye to be treated and also in the programmable features of the system. As shown in FIG. 1 of that patent, the anatomy of the portion of the limbal area to be treated is characterized by a curvilinear shape with a radius of approximately 7.5 mm. The arc lenght and width of treatment in the corneal scleral bed partially is determined by the severity of the glaucoma; the more severe glaucoma requires a broader and longer arc of trabeculodissection. The width is limited by the fact that the average maximum width of the trabecular meshwork is less than 1 mm. The arc is limited by the circumference of the limbal area around the eye.
Moreover, the trabecular mesh work is covered by a uneven amount of corneoscleral tissue. Specifically, the anterior most portion near the scleral septum is deeper and thinner than is the posterior portion near the scleral spur (iris root). The latter portion is more superficial and thicker. In addition, the ablation rate of corneal tissue is different from scleral tissue.
The goal of laser trabecular dissection (LTD) is to achieve as wide an area (anteroposterior) as possible of partial thickness dissection over the trabecular meshwork, especially the posterior portion, sufficiently deep to allow for adequate aqueous drainage, but not so deep that the dissection enters the anterior chamber.
A galvanometric scanning delivery system (GSS) is ideally suited to meet the above objectives. Specifically, the laser is programmed for a low, but suprathreshold, power, typically less than 120 mj/cm.sup.2, to reduce the risk of full thickness penetration. The pulse frequency is set, typically at 60 to 300 Hertz, to achieve as rapid a rate of ablation as possible so as to reduce the potential for slow filtration that could absorb the laser energy and mask the laser effects. The GSS also allows selection of small spot size, in the range to 100 to 300 microns.
Variable aperture delivery system cannot be programmed to archive an appropriate ablation profile. The lack of a homogenous energy profile can create hot and cold spots and increase the risk of full perforation. Moreover, the attendant acoustic shock wave promotes premature drainage of aqueous that interferes with the ablation.
Furthermore, prior art laser trabeculodissection has an endpoint. When filtration begins, the aqueous humor absorbs the laser energy and masks the laser's effect. It would be beneficial, therefore, to use a method of surgery that allows filtration to proceed at one ablation site without interfering with the laser energy at a subsequent site.
As disclosed above, O'Donnell has used a galvanometric scanning delivery system to achieve trabeculodissection, and his method provides a plurality of test zones across the width and lenght of the treatment so that the system can be programmed to provide the optimum level dissection across a treatment area of variable thickness. O'Donnell performs the ablation in zones based upon the results of a test zone to obtain ablation without perforation and his method utilizes a relatively low power to reduce the risk of inadvertent full-thickness perforation into the anterior chamber.
The method of O'Donnell avoids acoustic shock waves, and the laser system has a homogenous energy profile and removes tissue in scan layers to process zones of minimal thickness.
Thus, it is well known from the prior art to provide a method of performing trabeculodissection using a galvanometric scanning laser delivery system. The surgeon uses a knife, such as a diamond knife and scleral dissector to make a scleral flap and expose the treatment arc of trabecular meshwork. The arc of the treatment area is as wide as the trabecular meshwork and lenght of the arc is limited by the circumference of the limbal area around the patient's eye. The surgeon uses the laser to treat small test areas in successive discrete zones along the arc of the treatment area in the bed of the scleral flap to determine the precise depth of ablation required over each entire zone so as to promote filtration without penetration of the treatment zone. The laser then is programmed to treat the length of the arc in discrete zones. Tissue is removed in scan layers, typically 2 microns thick, so as to process discrete ablated zones of minimal residual thickness. The treatment of successive zones allows ablation along the lenght of the treatment arc without interference from actively draining aqueous. After ablation of the various successive zones, the scleral flap is closed and, if necessary, sutured.
Between 1946 and 1949, Goldman was the first one to make a precise experiment to determine the place of the aqueous humor outflow resistance. He found that the place was the trabeculum. Grant performed perfusion experiments in enucleated human eyes between 1955-1958. Keeping up a continuous flow in the anterior chamber, he extirpated the trabeculum in 360.degree. at the Schlemm channel level to find out that resistance diminished in a 75%(S). Seiler, between 1985-1988, was the first to perform a partial trabeculectomy with an excimer laser. He learned that 94% of the resistance was in the last 10 microns of the yuxtacanalicular tissue (B).
In 1993, Arenas Archillas published the AB outer trabeculectomy procedure, that is, a manual trabeculodissection procedure and the direct precedent of laser trabecular dissection (LTD). Later on, he modified his own technique employing a diamond drill and adding 0.04 mg/cc Mitomycin (Highlight 216-226).
The literature leads to the conclusion that all surgical procedures tend to eliminate or reduce the aqueous outflow resistance. The most current glaucoma surgery employed at the moment is the trabeculectomy described by Cairns in 1968.
The glaucoma technique by surgery that is presently in use is the trabeculectomy. This surgical procedure consists of performing a conjunctival incision (fornix or limbar based), to free the sclera area near the sclera-corneal limbus. A lamellar scleral flap is (300 to 400 micron thick), which is rectangular, triangular or circular, with its base directed to the cornea. Hereafter, this flap is referred to as the scleral roof. Once the scleral flap is bent over the cornea, part of the deep sclera (scleral floor), which can be about 3.times.2 mm rectangular, is cut out to expose the trabeculum zone and the Schlemm channel (normal drainage that are in part or completely blocked). With this a fistul is established in the intrascleral space. After performing a basal iridectomy, the scleral flap is repositioned and sutured. The conjunctiva is also sutured.
With this procedure, a valved mechanism of filtration is established by means of which aqueous humor has free access throughout the anterior chamber, the intrascleral space and subjunctival space with the consequent regulation of intraocular pressure.
The disadvantages of this technique are that, since it is an intraocular procedure, the eye is abruptly decompressed in making the 2.5 to 3 mm opening. This may result in a serious surgical accident, such as vitreous loss and even an expulsive hemorrhage that may be responsible for surgical failure, or even total visual loss.